viki
05-09-2010, 01:17 AM
10. Create a Crow’s Foot ERD for each of the following descriptions. (Note: The word many merely means “more than one” in the database modeling environment.)
a. Each of the MegaCo Corporation’s divisions is composed of many departments. Each of those departments has many employees assigned to it, but each employee works for only one department. Each department is managed by one employee, and each of those managers can manage only one department at a time.
The Crow’s Foot ERD is shown in Figure T1.20A.
FIGURE T1.20A The MegaCo Crow’s Foot ERD
74
As you discuss the contents of Figure T1.20A, note the 1:1 relationship between the EMPLOYEE and the DEPARTMENT in the “manages” relationship and the 1:M relationship between the DEPARTMENT and the EMPLOYEE in the “is assigned to” relationship.
b. During some period of time, a customer can rent many videotapes from the BigVid store. Each of the BigVid’s videotapes can be rented to many customers during that period of time.
The solution is presented in Figure T1.20B. Note the M:N relationship between CUSTOMER and VIDEO. Such a relationship is not implementable in a relational model.
FIGURE T1.20B The BigVid Crow’s Foot ERD
75
If you want to let the students convert Figure T1.20’s ERD into an implementable ERD, add a third RENTAL entity to create a 1:M relationship between CUSTOMER and RENTAL and a 1:M relationship between VIDEO and RENTAL. (Note that such a conversion has been shown in the next problem solution.)
c. An airliner can be assigned to fly many flights, but each flight is flown by only one airliner.
FIGURE T1.20C The Airline Crow’s Foot ERD
76
d. The KwikTite Corporation operates many factories. Each factory is located in a region. Each region can be “home” to many of KwikTite’s factories. Each factory employs many employees, but each of those employees is employed by only one factory.
The solution is shown in Figure T1.20D.
FIGURE T1.20D The KwikTite Crow’s Foot ERD
7778
e. An employee may have earned many degrees, and each degree may have been earned by many employees.
The solution is shown in Figure T1.20E.
FIGURE T1.20E The Earned Degree Crow’s Foot ERD
78
Note that this M:N relationship must be broken up into two 1:M relationships before it can be implemented in a relational database. Use the Airline ERD’s decomposition in Figure T1.20C as the focal point in your discussion.
a. Each of the MegaCo Corporation’s divisions is composed of many departments. Each of those departments has many employees assigned to it, but each employee works for only one department. Each department is managed by one employee, and each of those managers can manage only one department at a time.
The Crow’s Foot ERD is shown in Figure T1.20A.
FIGURE T1.20A The MegaCo Crow’s Foot ERD
74
As you discuss the contents of Figure T1.20A, note the 1:1 relationship between the EMPLOYEE and the DEPARTMENT in the “manages” relationship and the 1:M relationship between the DEPARTMENT and the EMPLOYEE in the “is assigned to” relationship.
b. During some period of time, a customer can rent many videotapes from the BigVid store. Each of the BigVid’s videotapes can be rented to many customers during that period of time.
The solution is presented in Figure T1.20B. Note the M:N relationship between CUSTOMER and VIDEO. Such a relationship is not implementable in a relational model.
FIGURE T1.20B The BigVid Crow’s Foot ERD
75
If you want to let the students convert Figure T1.20’s ERD into an implementable ERD, add a third RENTAL entity to create a 1:M relationship between CUSTOMER and RENTAL and a 1:M relationship between VIDEO and RENTAL. (Note that such a conversion has been shown in the next problem solution.)
c. An airliner can be assigned to fly many flights, but each flight is flown by only one airliner.
FIGURE T1.20C The Airline Crow’s Foot ERD
76
d. The KwikTite Corporation operates many factories. Each factory is located in a region. Each region can be “home” to many of KwikTite’s factories. Each factory employs many employees, but each of those employees is employed by only one factory.
The solution is shown in Figure T1.20D.
FIGURE T1.20D The KwikTite Crow’s Foot ERD
7778
e. An employee may have earned many degrees, and each degree may have been earned by many employees.
The solution is shown in Figure T1.20E.
FIGURE T1.20E The Earned Degree Crow’s Foot ERD
78
Note that this M:N relationship must be broken up into two 1:M relationships before it can be implemented in a relational database. Use the Airline ERD’s decomposition in Figure T1.20C as the focal point in your discussion.